Diet rich in antioxidants can cut heart attack risk

10:20Sunday 23 September 2012

An “antioxidant-rich diet ‘cuts heart attack risk’,” The Daily Telegraph has reported. It goes on to say that older women who ate “seven fruit and vegetable portions a day were between 20 and 29 per cent less likely to have a heart attack over a decade than those who ate just 2.4 [portions]”.

This news is based on a large study that included more than 30,000 women who were free of heart disease. The researchers asked participants about their diets and looked at whether they had a heart attack over the following 10 years. The researchers also estimated the amount of antioxidants in the women’s diets and whether this was associated with their risk of having a heart attack.

They found that women who had the highest levels of antioxidants in their diets (those who ate six or more portions of fresh fruit and vegetables a day) were 20% less likely to suffer a heart attack over 10 years compared with women who ate the lowest levels.

Sadly, for those of us looking for a quick fix, previous research has failed to find a similar preventative effect in people taking vitamins and supplements rather than eating a healthy balanced diet. The researchers speculate that it is not the amount of antioxidants consumed that is important but the range of different food sources from which the antioxidants come that helps protect health.

This study does not conclusively prove that the antioxidants in fruit, vegetables and wholegrains are good for you. It simply confirms that eating plenty of fruit and vegetables makes you less likely to die from a heart attack.

Antioxidants are chemicals found in certain foods such as leafy green vegetables, fresh fruit and wholegrains. It is thought that they may help protect against the harmful effects of atoms and molecules known to cause cell damage (free radicals).

The study was carried out by researchers from the Karolinska Institutet in Sweden as well as the University of Alabama at Birmingham and the Beth Israel Deaconess Medical Center in the USA. The research was funded by the Swedish Research Council for Infrastructure and the Swedish Council for Working Life and Social Research.

The study was published in the peer-reviewed American Journal of Medicine.

The Telegraph provided an accurate overview of the research, although the headline did not make clear that the research only involved women. While a reasonable assumption can be made that similar results would be found in men, this cannot be proven by the research under discussion.

This was a prospective cohort study that examined the association between total amounts of dietary antioxidants and the risk of having a heart attack over 10 years. Antioxidants are compounds that are thought to play a role in protecting against heart disease by interfering with molecules known as ‘free radicals’.

Free radicals are molecules known to cause cell damage and are associated with the negative effects of ageing, as well as exposure to harmful substances such as high levels of radiation.

Free radicals are thought by some to cause damage to blood vessels, which can disrupt the flow of blood to the heart (coronary heart disease). Coronary heart disease can increase the risk of a blockage occurring in a blood vessel, which can cause a blood clot to form, blocking the supply of blood to the heart and triggering a heart attack.

The researchers report that a diet rich in fruit and vegetables, which contain high levels of antioxidants, has been shown to be associated with a reduced risk of coronary heart disease.

However, supplements that contain a high dose of a single antioxidant have shown conflicting results. Some research has shown that antioxidant supplements do not protect against heart disease (or other conditions) and can perhaps increase risk of dying. For example, the use of the antioxidant supplement beta carotene, long thought to provide protection against cancer, was found to increase the risk of lung cancer in smokers.

The researchers suggest that consuming antioxidants from a wide range of dietary sources, such as leafy green vegetables, fresh fruit and wholegrains is better than relying on supplements. Increasing the range may allow different types of antioxidants to interact with each other, providing an increased level of protection.

To test their theory the researchers assessed the diets of thousands of women, estimated the total amount of antioxidants consumed and assessed whether or not these levels were associated with differing risk of heart attack over the course of a decade.

In 1997, researchers sent questionnaires to a group of 56,030 women who had previously enrolled in another study (the Swedish Mammography Study). Of these women, 38,984 (70%) returned the questionnaires, providing data on their diets, lifestyles and medical history. After excluding some women with cancer, diabetes and cardiovascular disease, the researchers narrowed the study cohort down to 32,561 middle-aged and elderly women.

These women completed a 96-item questionnaire that asked how often they consumed a certain type of food or drink. The researchers used this information to estimate their average total dietary antioxidant intake. They then ranked the intake of the cohort, and divided it into four separate groups, from highest antioxidant intake to lowest. The researchers followed-up with the cohort, using Swedish national records to identify which of the women had a heart attack over the next 10 years. They compared the risk of having a heart attack among those who consumed the most antioxidants to the risk among women who consumed the fewest.

The researchers also collected data on a number of other factors that may affect risk of heart disease, including body mass index (BMI), physical activity levels, smoking status and age. These factors were taken into account in the data analysis to avoid confounding of the results.

During the follow-up period, the researchers found that there were 1,114 heart attacks among the women.

After adjusting for potential confounding factors, women with the highest total antioxidant intake at the beginning of the study had a 20% lower risk of heart attack over the follow-up period compared with women with the lowest intake (hazard ratio 0.80, 95% confidence interval 0.67 to 0.97).

The researchers concluded that inclusion of higher levels of antioxidants in the diet was associated with a significantly lower risk of having a heart attack.

This study suggests that a diet rich in fruit and vegetables, which are a good source of antioxidants, may reduce the risk of having a heart attack among middle-aged and elderly women.

This study has several strengths. It was a large study in more than 30,000 women, and it collected data at the beginning of the study and then followed-up over a fairly long period of time. This allows us to be more confident that the results reflect real relationships, and the large number of women included from the general population improves our ability to generalise the results to women who did not participate in the study.

There are some limitations to the study that should be considered, however. The most important is that researchers collected dietary data at one point in time only. There is no way to be sure that the dietary habits of the women did not change over the 10-year follow-up period. If the habits did change, this could introduce bias into the study. Additionally, the study relied on self-report to measure diet, which could also introduce bias if the women did not accurately report their dietary habits.

Another limitation is that, as you would expect, women who ate a lot of antioxidants in the form of fresh fruit and vegetables were more likely to live healthier lifestyles (such as not smoking and taking regular exercise) than women who didn’t. So, while the evidence provided by the study is compelling, it is not possible to infer a direct cause and effect relationship between diet and risk of heart attack.

One strength of the study is that the researchers’ method of measuring antioxidant intake included measures of how various dietary sources of antioxidants may interact. This was assumed to give a better measure of the beneficial effects of an antioxidant-rich diet than simply measuring overall antioxidant consumption.

They say that fruit and vegetable intake accounted for 44% of the dietary antioxidants among the cohort. Other foods that contributed were wholegrains and, to a lesser extent, coffee.

Overall, this study, perhaps unsurprisingly, suggests that eating a lot of fruit, vegetables and wholegrains is good for your heart.

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